This invention relates to a method and means for automatically controlling continuously operating chemical reactors and more particularly for controlling and improving the efficiency of membrane-type chlor-alkali cells.
In energy intensive processes, such as the electrolytic production of caustic soda solution and chlorine and hydrogen gases in membrane chlor-alkali cells, it is critical that overall operating efficiency be continually improved if a commercially competitive position is to be maintained. To do this, there has been a major effort to design and produce new, improved cell structures, dimensionally stable anodes, catalytic low overvoltage cathodes and high performance membranes, all of which act to lower power consumption. However, unless careful control is exerted over all aspects of the operation of such cells, the cost benefits obtained by such improvements can quickly be lost.
It is known in the art that the overall efficiency of a membrane-type chlor-alkali cell, as measured by the number of kilowatt hours required per unit of caustic produced, is a complex resultant of the interaction of a number of factors. These include, among other things, the basic design of the cell, the nature and structure of the anodes and cathodes used, the water and cation transport characteristics of the membrane, the concentration, pH, temperature and flow rate, or residence time, of the anolyte brine and catholyte caustic solutions within the cell and the cell current and voltage. While a number of these factors are essentially fixed once the cell is assembled and placed into operation, others, primarily related to the electrical and fluid-flow aspects, are capable of considerable and sometimes unpredictable changes during cell operation. Whenever such changes occur, it is usually necessary to correct them as quickly as possible if the system is to be restored to the level of efficiency previously obtained with minimum cost penalties.
While past experience often provides a guide as to what action, and how much of it, should be taken, the operating characteristics of a modern large multicell system are such that either the cause or the effects of an "upset" must usually be fairly massive before it is detected. Consequently, whatever changes are applied usually take fairly substantially periods of time before they are fully effective. Thus, it is difficult, if not impossible, for an operator to detect such a problem, analyze its significance and then interact with the system in a manner most likely to correct the problem in the shortest possible time. Moreover, several attempts may be required before full system efficiency is recovered. This is especially true in plants wherein a large number of cells are interconnected to increase product output. Further, even without an operational problem, the overall complexity of such a system tends to make it quite difficult for human operators to determine if both the individual units and the total system are all operating at maximum efficiency at any precise time. This is particularly true, where whatever changes are occurring, are the result of a slow, continuous degradation of one or more of the system components.